GregBBRD

Amazingly enough, the Workshop manual says that you can do this very thing!

Overbore one cylinder and leave the rest alone! You only need to make sure that the oversize piston weighs what the original ones do.

Say what?

The difference in "actual cylinder volume" is very small....certainly less than the average "leakdown" of most engines....so the tiny amount of volume difference is a moot point!

Mrmerlin

LFA where are you typing these posts from?
What city does the mechanic that honed you a fine hone job work in?

Bill Ball

Several years ago George Suennen had his engine rebuilt by a shop that should have known better. The rebuilt engine did not run very well and we found leakdown of 12%. (My unopened stock S4 motor had 3% leakdown at 150K miles.) We were told by the shop that 12% was "not bad". Uhuh. And it only got worse. A few months later we measured the leakdown at 25% in several cylinders. On taking the engine apart, cross-hatching was clearly visible on the cylinder walls, among a number of other signs of a completely FUBAR'd rebuild.

jeff spahn

I say + 928 gazillion on this!

LFA

Guys, I now live in Istanbul Turkey. I lived and studied Champaign-Urbana İllinos, had my graduate degree there, worked in London and have been living in my home country. There are only 8 late model ( 1997-1991) 928s here and I bought 5 of them in the last year and recently started rebuilding them. They are 1987, 88, 89, 90 S4s and one 1991 GT. The first project was the 1991 GT and unfortunately started with this disaster. The body is an empty shell now, and I am buying a lot of parts from Germany and UK. My mechanic is an experienced guy in Porsche 997, 996, 991, Nissan GTRs and Supras but he failed big time in this. I very much appreciate all your help and comments. They have been very helpful.
Since yesterday I have been researching what is available to properly hone the Alusil block.

LFA

While trying to find the tolerance group I found the following numbers on the block: 59 04 D.

Jim Devine

This is from Sunnen, makers of honing equipment:

http://www.sunnen.com/NewsDetails.aspx?NewsID=11

Mrmerlin

thanks for the additional info on where your located .

If you go back to post 9 you will see a picture of the block ,
you will see that there is a number just above each cylinder this is the tolerance group and the piston will have a matching number.

Since one bore has been damaged, you will possibly be able to refinish that bore and fit a different tolerance piston if the damage isnt too bad.

Of course if you could post pictures of the damage and what was the reason the honing operation was carried out in the first place,
that would help to explain how to proceed.
IE there was a gouge in the cylinder bore.
The more info you provide the better your responses will be,
and the easier it will be to assist you in repairing this engine

ptuomov

Run away from the mechanic who has just destroyed your original GT engine.

Buy a used S4 engine from a rear-ended car from Ebay. They go for about $2500 with accessories. Then, swap the heads and intake from your GT engine to that S4 engine. That might get you there with the least bit of aggravation.

namasgt

Like Stan said post a picture to see how bad he honed it. Also, not sure who your getting your parts from in Germany but it may end up being cheaper buying them from one of the vendors here and shipping them over.

https://rennlist.com/forums/928-foru...in-europe.html

namasgt

One more thing, the body on these cars have zinc to prevent corrosion. When your stripping the paint take that into consideration.

Here is a thread of someone here restoring a GT from empty shell that you may find interesting:

https://rennlist.com/forums/928-foru...ished-car.html

LFA

Thanks,your comment and that tread are very helpful

namasgt

On your other thread you asked about optimization. The only thing I can advice for your other cars is to change all the old vacuum lines, seals, knock sensors to get the engine to the factory spec. Also do a timing belt job as it needs to be done every 5 years or 40-50K miles, these engines are interference and if the belt breaks you will end up with bent valves. On the 87+ cars they used knock sensors to retard the timing during a detonation event. The knock sensors retard the timing to prevent damage to the engine in the event of detonation. Once the knock sensors go bad due to age the computer retards the timing and the car will be under powered by a noticeable amount until you install new knock sensors. Also the intake develops leaks from aged vacuum lines and flapper bearing seals that also contribute in reducing performance. Here is a write up from Dwayne for intake refresh, note that I would drain the coolant from the block before making an attempt in unbolting the intake (there are two coolant drain plugs, one on each side, back lower of the block, work shop manual has a picture of these).

Intake refresh:
https://rennlist.com/forums/928/4826...re-w-pics.html

Dwayne's garage:

http://dwaynesgarage.norcal928.org/

928 DIY repairs:

https://rennlist.com/forums/928-134/


Also, if any of your cars is an automatic, you want to make sure you check and release the flex plate, because if you don't release the pressure you could end up with a cracked non fixable block:

https://rennlist.com/forums/928-foru...pictorial.html

https://rennlist.com/forums/928-foru...re-w-pics.html

You want to also make sure all of the old coolant is completely flushed out from your other cars and replaced with the correct coolant. Old coolant turns acidic and eats up the Alusil block and will create holes on the cylinder walls. Also change all the fuel lines with factory fuel lines or the ones Greg Brown (GregBBRD) makes (Most of the factory ones are not available anymore). Too many 928s have burned to the ground due to old fuel lines.

Here is some pictures of the correct fuel lines you need:

https://rennlist.com/forums/928-foru...e-bulging.html

As for your current problem on the GT, talk to one of the UK guys like John Speak to see if they know someone that does cylinder boring work on Alusil blocks. You can probably over bore all 8 and go with the much bigger 968 pistons as they have the correct coating, you will end up with 104mm bores instead of 100mm I believe. But I'm not sure if you can reuse the 5 liter crank shaft, may need a crank with longer stroke... You may be able to find two sets of Porsche 968 pistons of the same tolerance group for much less money than new ones from Porsche.

I would still consider sending the block and one of the pistons to US to have the bores Nikasil coated. May cost from $2000 to $3000 to do it.

namasgt

Also, if one of your cars doesn't start or is not running properly, make a thread and post the symptoms before you or a mechanic attempts to fix anything. We may save you a few bucks.

This is very helpful and as Randy suggested take the time to read it...

https://rennlist.com/forums/928-foru...questions.html

LFA

Thanks for your replies. Now I am trying to understand the condition of the untouched 7 cylinders and their pistons. What is the maximum allowable difference between the cylinder diameter and the piston diameter for a used car.
The workshop manual says the normal difference is 0.020 mm for a new part (such as piston 99.980mm, cylinder 10.000mm). At which level you need to replace the pistons?. Thanks